1. Field of the Invention
The present invention relates to an image processing apparatus which converts a multiple gradation image into a binary or multivalue image by use of an area gradation method according to a screen cell, and more particularly to an image processing apparatus, which performs halftone processing to realize fine expression with multiple gradation according to dot growth properties that screen cell dots increase or decrease with an increase in gradation degree, and an image processing method.
2. Description of the Related Art
For example, for halftone processing (halftoning) of a multiple gradation image of a laser printer, an area gradation method is extensively used to array prescribed screen cells to form a multiple gradation image and to largely grow dots in a prescribed order with an increase in gradation value.
Incidentally, in a device with poor reproducibility of isolated dots such as a laser printer, it is possible to improve reproducibility of a low gradation portion and to improve the definition of intermediate and high gradation portions by configuring a screen cell to have large dots with a small line number in the low gradation portion and a screen cell to have fine dots with a large line number in the intermediate and high gradation portions.
FIG. 13A through 13F are diagrams showing a screen cell used for conventional halftone processing and dot growth thereof.
FIG. 13A through 13C show that a screen cell 60 with a small line number [FIG. 13A] is used, individual dots in the screen cell 60 grow in a prescribed order with an increase in the gradation value of individual pixels of multiple gradation image data to be converted, and when a dot shape with a prescribed gradation value is determined [FIG. 13B], the pertinent dot shape is remained as it is when a gradation value is higher than it, and the dots further grow [FIG. 13C].
Similarly, FIG. 13D through 13F show that a screen cell 70 with a large line number [FIG. 13D] is used, and when a dot shape with a prescribed gradation value is determined [FIG. 13E] in a process of growing individual dots in the screen cell 70 according to a prescribed order with an increase in the gradation value of individual pixels, the dot shape is remained as it is when a gradation value is higher than it, and the dots further grow [FIG. 13F].
Thus, according to the conventional known halftone processing technology, the dots in the screen cell grow according to the prescribed order, and when a dot shape with a prescribed gradation value is determined, the dot shape is remained as it is when the gradation value is higher than it, and the dots grow furthermore.
According to the above method, the dot shape which has become large cannot be made smaller. Therefore, it is impossible to switch a screen line number according to a prescribed gradation value by using one screen cell in such a way that the low gradation portion is based on a small line number screen, and the intermediate and high gradation portions are based on a large line number screen.
Therefore, there are developed various technologies that a screen cell with plural line numbers is provided in advance and switched.
Japanese Patent Laid-Open Publication No. 2000-71439 describes a technology that in a multivalue printer having plural kinds of dots such as small dots and large dots, such as an inkjet printer, has a highlight portion interlocked with the small dots and made to come interlocking with the large dots as it becomes thick.
Japanese Patent Application Laid-Open No. 11-331562 discloses a technology that a screen is switched between a small line number and a large line number depending on the image properties (edge/non-edge).
Japanese Patent Application Laid-Open No. 11-331562, No. 7-254985, No. 7-254986, No. 7-283941, No. 8-114965, No. 8-125863 and No. 8-156329 disclose technologies that to improve reproducibility of a highlight portion, pulse width modulation is used for writing, and dither processing with 2 dots weighted in a main scanning direction is performed to stabilize the highlight portion by reproduction with a small line number.
Especially, Japanese Patent Application Laid-Open No. 8-156329 discloses a technology that it is judged whether a kind of image density signal is a line image or a natural image, and an optimum recording line number is selected depending on the kind.
According to the existing technologies described in Japanese Patent Application Laid-Open No. 2000-71439, No. 11-331562, No. 7-254985, No. 7-254986, No. 7-283941, No. 8-114965, No. 8-125863 and No. 8-156329 described above, plural types of screen cells are provided in advance, the screen cells are switched depending on the image properties (characters/photos, edges/non-edges and the like) or depending on low density or high density.
Therefore, it is necessary to provide previously the plural types of screen cells, and it is necessary to have a mechanism of switching the individual screen cells.
According to the above conventional method, the screen cell numbers to be switched are variable depending on the printer properties and an object, and there may be a problem that flexible compliance cannot be made by a prescribed number of screen cells.
An increase in the types of screen cell numbers subject to switching increases the storage capacity of the screen cell and makes the switching control complex, resulting in increasing the apparatus cost.